B lymphocyte activation is critical in the generation of adaptive immune responses. Derailed B lymphocyte activation is a hallmark of many autoimmune diseases and modulation of this immune response is therefore of therapeutic interest. Recently, the success of B cell therapies in autoimmune diseases has been established. Treatment of rheumatoid arthritis (RA) patients with rituximab (anti-CD20 therapy) is an accepted clinical therapy. More recent clinical trial studies show that treatment with rituximab also ameliorates disease symptoms in relapsing remitting multiple sclerosis (RRMS) and systemic lupus erythematosus (SLE) patients. This success supports the potential for future therapies in autoimmune diseases targeting B cell immunity.
Bruton's tyrosine kinase (BTK) is a Tec family non-receptor protein kinase expressed in B cells and myeloid cells. The function of BTK in signaling pathways activated by the engagement of the B cell receptor (BCR) and FcεR1 on mast cells is well established. In addition, a function for BTK as a downstream target in Toll like receptor signaling is suggested. BTK is composed of the pleckstrin homology (PH), Tec homology (TH), Src homology 3 (SH3), Src homology 2 (SH2), and tyrosine kinase or Src homology 1 (TK or SH1) domains. The function of BTK in signaling pathways activated by the engagement of the B cell receptor (BCR) in mature B cells and FCER1 on mast cells is well established. Functional mutations in BTK in humans result in a primary immunodeficiency disease (X-linked agammaglobuinaemia) characterized by a defect in B cell development with a block between pro- and pre-B cell stages. The result is an almost complete absence of B lymphocytes, causing a pronounced reduction of serum immunoglobulin of all classes. These findings support a key role for BTK in the regulation of the production of auto-antibodies in autoimmune diseases.
BTK is expressed in numerous B cell lymphomas and leukemias. Other diseases with an important role for dysfunctional B cells are B cell malignancies, as described in Hendriks, et al., Nat. Rev. Cancer, 2014, 14, 219-231. The reported role for BTK in the regulation of proliferation and apoptosis of B cells indicates the potential for BTK inhibitors in the treatment of B cell lymphomas. BTK inhibitors have thus been developed as potential therapies for many of these malignancies, as described in D'Cruz, et al., OncoTargets and Therapy 2013, 6, 161-176. With the regulatory role reported for BTK in FccR-mediated mast cell activation, BTK inhibitors may also show potential in the treatment of allergic responses, as described in Gilfillan, et al., Immunologic. Rev. 2009, 288, 149-169. Furthermore, BTK is also reported to be implicated in RANKL-induced osteoclast differentiation, as described in Shinohara, et al., Cell 2008, 132, 794-806, and therefore may also be of interest for the treatment of bone resorption disorders. Other diseases with an important role for dysfunctional B cells are B cell malignancies. Indeed anti-CD20 therapy is used effectively in the clinic for the treatment of follicular lymphoma, diffuse large B-cell lymphoma and chronic lymphocytic leukemia, as described in Lim, et al., Haematologica, 2010, 95, 135-143. The reported role for BTK in the regulation of proliferation and apoptosis of B cells indicates there is potential for BTK inhibitors in the treatment of B cell lymphomas as well. Inhibition of BTK seems to be relevant in particular for B cell lymphomas due to chronic active BCR signaling, as described in Davis, et al., Nature, 2010, 463, 88-94.
In many solid tumors, the supportive microenvironment (which may make up the majority of the tumor mass) is a dynamic force that enables tumor survival. The tumor microenvironment is generally defined as a complex mixture of “cells, soluble factors, signaling molecules, extracellular matrices, and mechanical cues that promote neoplastic transformation, support tumor growth and invasion, protect the tumor from host immunity, foster therapeutic resistance, and provide niches for dominant metastases to thrive,” as described in Swartz, et al., Cancer Res., 2012, 72, 2473. Although tumors express antigens that should be recognized by T cells, tumor clearance by the immune system is rare because of immune suppression by the microenvironment. Addressing the tumor cells themselves with e.g. chemotherapy has also proven to be insufficient to overcome the protective effects of the microenvironment. New approaches are thus urgently needed for more effective treatment of solid tumors that take into account the role of the microenvironment.
Some of the BTK inhibitors reported to date are not selective over Src-family kinases. With dramatic adverse effects reported for knockouts of Src-family kinases, especially for double and triple knockouts, this is seen as prohibitive for the development of BTK inhibitors that are not selective over the Src-family kinases. Both Lyn-deficient and Fyn-deficient mice exhibit autoimmunity mimicking the phenotype of human lupus nephritis. In addition, Fyn-deficient mice also show pronounced neurological defects. Lyn knockout mice also show an allergic-like phenotype, indicating Lyn as a broad negative regulator of the IgE-mediated allergic response by controlling mast cell responsiveness and allergy-associated traits, as described in Odom, et al., J. Exp. Med., 2004, 199, 1491-1502. Furthermore, aged Lyn knock-out mice develop severe splenomegaly (myeloid expansion) and disseminated monocyte/macrophage tumors, as described in Harder, et al., Immunity, 2001, 15, 603-615. These observations are in line with hyperresponsive B cells, mast cells and myeloid cells, and increased Ig levels observed in Lyn-deficient mice. Female Src knockout mice are infertile due to reduced follicle development and ovulation, as described in Roby, et al., Endocrine, 2005, 26, 169-176. The double knockouts Src−/−Fyn−/− and Src−/− Yes−/− show a severe phenotype with effects on movement and breathing. The triple knockouts Src−/− Fyn−/− Yes−/− die at day 9.5, as shown by Klinghoffer, et al., EMBO 1, 1999, 18, 2459-2471. For the double knockout Src−/− Hck−/−, two thirds of the mice die at birth, with surviving mice developing osteopetrosis, extramedullary hematopoiseis, anemia, leukopenia, as shown by Lowell, et al., Blood, 1996, 87, 1780-1792. Hence, an inhibitor that inhibits multiple or all kinases of the Src-family kinases simultaneously may cause serious adverse effects.